Mobile communication systems are in constant development. Even at the present there are several different mobile communication systems in use, which operate overlapping each other in the same area. At present one of the most frequently used networks is the GSM (Global System for Mobile telecommunications) network, which still will be used a long time, due to its high degree of utilisation and prevalence. New network topologies will rise in parallel with this network, and typically the utilization of these new topologies require new terminals. In future it must be possible to use the terminals in a number of networks. Then a change over from the use of a first network to the use of a second network must be effected without problems.
Particularly at present there is a general opinion that at least some of the future mobile communication networks will be based on the so called wideband CDMA technique (WCDMA; Wideband Code Division Multiple Access). The WCDMA technique is at present applied for instance in the UMTS system (Universal Mobile Telecommunication System). Thus it is important that the terminals in for instance the WCDMA system can change over to use for instance a GSM connection when required. The change over from using one network to another is usually called a inter system handover.
A terminal may be forced to change over from one network to another for instance when the terminal gets outside the coverage area of that first network to which it is connected. Then, if the coverage area of a second network is sufficient at the terminal's new geographical location, it is advantageous to change over to use said second network system. In such a case the change over from using one network to using a second network can advantageously be made with the same actions as in a handover between base stations in the same network.
The answer to the question when one shall change over to use the second network is not unambiguous. According to a first idea a terminal blindly changes over to use the second network arrangement when the reception from that network, to which the terminal originally was linked, is disconnected. According to another idea it is possible to make measurements between the networks, in the same way as in present network solutions which use the TDMA system (Time Division Multiple Access). According to this arrangement the connection quality is measured, and if it is inadequate, then terminal changes over to use the second network.
The problem in the above presented first solution is that if the terminal blindly decides to change over to use the second network, then it is not completely sure that the reception from the second network is at all better than that of the currently used network. If the reception from the new network is weaker, or if there is no reception at all, the terminal loses for a moment the connection to both possible networks. Thus a blind change of network can be considered as a very unreliable alternative. The arrangement of the second prior art solution is in other respects acceptable, but it is not suited to be used in all network systems, such as in the WCDMA system (Wideband Code Division Multiple Access). This is due to the fact that a measurement made in the TDMA system at handover will interfere with the operation of the WCDMA system and reduce the capacity of said system.
It should be particularly noted that the above presented solutions are intended to be applied in a situation where the first and second networks have similar systems, i.e. that both are for instance GSM networks. They may be for instance maintained by different operators having a mutual contract about roaming between each other's networks. Thus it is not possible to directly apply said solutions in a case where the concerned networks differ from each other regarding the technique.
Patent application WO9731503 discloses a cellular telephone system offering both CDMA and GSM services. In this system specific CDMA pilot beacons, which generate identifiable CDMA pilot signals, are located near or at GSM base stations. The edge of the CDMA service region is thus indicated by these pilot beacons. A subscriber unit detects the CDMA pilot signals and informs a base station controller of the CDMA system about them. The base station controller identifies the CDMA pilot signals as signals relating to the CDMA pilot beacons and initiates a CDMA to GSM handover.